This invention generally relates to an apparatus for coating articles or parts and, particularly, to an apparatus for spray coating continuously advancing articles in an enclosed atmosphere.
Generally, it is known to spray paint or coat articles by an apparatus having a conveyor which moves parts to and through some form of spraying booth where one or more nozzles coat the articles with a coating composition. These apparatus or systems are limited by the amount of volatile organic compounds that the sprayed coatings emit, the amount of floor space required to cure the coatings and by limited transfer efficiency.
For instance, the paint or coating composition, itself, creates considerable problems. If the composition uses components which are dispersed in various solvents or dilutents, the solvents or dilutents not only present the potential for explosion, but the amount of volatile organic components that can be emitted are highly regulated by governmental agencies. Water-based coatings may be used with less volatile organic components, but other complications such as the raising of wood grains must be overcome. Both solvent and water based coatings require long flash times to allow the solvents or dilutents to evaporate. This requires a considerable amount of floor space. In addition, these solvents or dilutents must be evacuated to atmosphere. Replacing the evacuated air is expensive, especially in colder climates or where buildings must be air conditioned.
In order to avoid the problems described above, ultraviolet (UV) curable coating compositions have been used. A 100% solids UV curable composition is a liquid composition and is void of solvents or dilutents which must be driven off in a curing or drying process. A UV composition cures only when exposed to ultraviolet light. In essence, a UV composition changes state from liquid to solid upon curing and there is no weight change between the two states. Unfortunately, UV compositions present their own set of problems of non-uniform coating thickness and an unnatural xe2x80x9cplasticxe2x80x9d look on wood articles. Such UV coatings also are difficult to apply to three-dimensional or contoured articles, particularly where the articles have leading and trailing edges to be coated. Prior apparatus have been designed to continuously advance articles through the apparatus while cyclically traversing the spray nozzles generally perpendicular to the movement of the articles. While such techniques may apply a more uniform coating on contoured articles, they create considerable problems in partially overlapped patterns which create transverse xe2x80x9cstripsxe2x80x9d of varying thickness. Roll coating processes can be used, but roll coating is limited to flat articles and cannot be used with three-dimensional articles such as contoured cabinet doors. Vacuum coating processes can be used, but such processes are limited to continuous profile articles such as moldings and cannot be used with articles having leading and trailing edges such as contoured cabinet doors. The terms xe2x80x9cleadingxe2x80x9d and xe2x80x9ctrailingxe2x80x9d edges of an article not only mean the first or front edge and the last or rear edge, respectively, of an article. Some articles, such as cabinet doors, have inside profiles which form depressed or recessed areas of the article between the front and rear edges of the article. These recessed areas form leading and trailing edges which extend transverse to the direction of movement of the article through the coating apparatus. A sphere is comprised almost entirely of a leading (hemispherical) edge and a trailing (hemispherical) edge.
A major problem with such coating apparatus is their inability to apply a thin-enough coating using 100% solids UV curable compositions on such articles as wood products to obtain a desired natural finish. This often is called a xe2x80x9cplasticxe2x80x9d look. This occurs because the spray nozzles are limited in their translatory speed. Since no solvents evaporate in 100% solids UV curable compositions, in order to obtain the same dry thickness, a much thinner coating must be applied. Some prior art systems have compromised in adding a solvent to dilute the UV composition. However, this solvent must be removed from the coating before it is cured, resulting in the above problems of volatile organic compounds, long flash tunnels, etc.
Still another problem with prior apparatus is the limited recovery of overspray. Systems that use solvent-based or water-based coating compositions typically are able to recover little or no overspray.
Systems that use UV coating compositions that are diluted with solvent are able to capture material deposited on the conveyor belt, but are able to respray the recovered coating only after more solvent is added to the composition. No known systems are able to collect both the overspray and material deposited on the belt and to respray the recovered coating without reformation.
The present invention is directed to solving this myriad of sometimes interrelated problems in a coating apparatus which incorporates a number of features which, in combination, produces an extremely high quality coating and even an unexpected finish on difficult articles to be coated.
An object, therefore, of the invention is to provide a new and improved apparatus for coating a continuously advancing article.
In the exemplary embodiment of the invention, the apparatus includes a housing defining a spray chamber having an inlet end and an outlet end. Wall means form an inlet buffer zone adjacent the inlet end and an outlet buffer zone adjacent the outlet end, with a spraying zone between the inlet and outlet buffer zones. Conveying means are provided for transporting articles to be coated through the spray chamber from the inlet to the outlet ends thereof. A plurality of spray guns are mounted on the housing in an array about the spraying zone to substantially surround an article on the conveying means transported through the spray chamber. The spray guns are adapted for spraying up to a 100% solids ultraviolet curable coating composition. Substantially the entirety of the spray guns are located outside the spray chamber, with only nozzle portions of the spray guns located inside the spray chamber, thereby reducing the size of the spraying zone.
As disclosed herein, the housing includes a cover defining the top of the spraying zone. The spray guns are mounted on the cover, and power means may be provided for raising and lowering the cover. The wall means which form the inlet and outer buffer zones with the spraying zone therebetween, comprise walls which extend downwardly from the cover to points short of the conveying means to allow the articles to pass beneath the walls. Drip troughs are provided at the bottom edges of the walls to prevent any coating composition accumulating thereon from dripping downwardly therefrom.
According to one aspect of the invention, the housing defines a center-line extending between the inlet and outlet ends and generally equidistant from opposite sides of the spray chamber. A plurality of the spray guns are mounted on each opposite side of the center-line, with the spray guns oriented to spray the spraying composition angularly inwardly relative to the center-line.
In addition, the housing defines a mid-point spaced between the inlet and outlet ends, and a plurality of the spray guns are mounted on each opposite side of the mid-point in directions toward and away from the inlet and outlet ends. The spray guns are oriented to spray the spraying composition angularly inwardly relative to the mid-point.
The conveying means may be provided by an endless conveyor belt having an upper run or surface which forms the bottom of the spray chamber. The conveyor belt can be moved at a relatively fast speed of 50-300 feet/minute to provide varying thicknesses of the coating composition.
With the spray guns located substantially entirely outside the spray chamber, the spray guns have extension portions projecting through the top of the cover to locate nozzle portions of the spray guns within the spray chamber. The nozzle portions are located approximately 8-30 inches from an article transported beneath the nozzle portions by the conveyor.
Finally, exhaust means are provided in communication with the spray chamber. The invention contemplates that the exhaust means be in communication with the buffer zones at opposite ends of the spraying zone within the spray chamber. In addition, a controlled amount of air is allowed to be exhausted from the spraying zone.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.